The Program Project for marrow transplantation in leukemia and genetic blood disorders is an integrated multidisciplinary program of basic and clinical research aimed at elucidating genetic, biologic and pharmacologic factors which influence the development of transplanted hematopoietic and lymphoid progenitor cells within an allogeneic host, and the donor and host cell interactions which foster or prevent engraftment, hematopoietic and immunologic reconstitution, and the emergence of graft-host tolerance, or GvHD, or relapse of leukemia. Specific objectives of the Program Project include: 1) Evaluation of T-cell depleted marrow transplants and their potential to abrogate GvHD and improve disease-free survival in both HLA-identical and HLA-disparate leukemic hosts. 2) Definition of cells, cell systems and cell products affecting the growth and development of transplanted hematopoietic progenitor cell populations depleted of T cells within an allogeneic host. 3) Development and evaluation of new therapies designed to foster the early reconstitution of resistance to infections within the transplanted host and 4) The development and evaluation of new cytoreductive regimens designed to more effective by eradicate leukemic cells and facilitate engraftment of allogeneic marrow cells while reducing damage to host cell systems necessary to sustain the patient and support the development of the marrow graft. Considerable progress has been made in each of these target areas.